An investigation of the effects of transforming growth factor-beta1 (TGF-beta1) on the proliferation of the various oncogene-transformed rat liver epithelial (RLE) cells indicated that all of these cell lines had acquired some resistance to the growth inhibitory effects of this cytokine. Analysis of binding of 125I-TGF-beta1 indicated that the less adherent more spindle shaped cell lines, which in turn were more tumorigenic, had lost receptors for TGF-beta1; however, the other resistant cell lines showed normal receptor binding, indicating that resistance occurs through post-receptor mediated events in the growth inhibition signal transduction pathway. Taken together with the findings of increased expression of TGF-beta1 in the more tumorigenic cell lines, these findings strongly indicate TGF-beta1 to be a major modulator of RLE cell phenotype. The possible role of other growth inhibitory polypeptides in the development of hepatocarcinogenesis is currently being investigated. A novel 20 KD liver-derived growth inhibitor (LDGI) has been purified to a high specific activity and its effects on the inhibition of proliferation of normal and transformed liver epithelial cells was compared with IL-6, which we have found to be a growth inhibitor for normal hepatocytes and RLE cells, and TGF-beta1. Our initial findings demonstrate that the transformed RLE show a loss of sensitivity to all of these growth inhibitors but to different degrees, suggesting that the growth inhibitory activities of these inhibitors are mediated through different signalling pathways and that these pathways may be comprised at different positions in the transformed cells. Thus, the development of blocks in growth inhibitory signaling pathways may be an important step in tumor progression.